Part of collection’s a breech-loading weapon (5)
I believe the answer is:
sabre
'weapon' is the definition.
(sabre is a kind of weapon)
'part of collection's a breech-loading' is the wordplay.
'part of' says the answer is hidden in the clue.
'SABRE' is hidden in the letters of 'collections a breech loading'.
(Other definitions for sabre that I've seen before include "Slasher" , "Sword (that may be rattled)" , "Type of sword" , "Large wasp" , "Walt --, cartoonist" .)